(-)-Galanthamine, and derivatives thereof, are useful for the treatment of Alzheimer's disease and related illnesses. Currently galanthamine is obtained by extraction from natural sources, such as daffodils or snowdrops. The yields of these extractive procedures are low, resulting in high costs and limited supplies of naturally-obtained galanthamine.
It is known that single enantiomer galanthamine (3) can be prepared from racemic narwedine (4) through resolution followed by reduction of the enone function, as depicted in Scheme 1 below. Usefully, since the enantiomers of narwedine (3) readily equilibrate (racemize) by way of reversible ring opening to a dienone, coupled to the fact that crystals of racemic (4) exist as a conglomerate of enantiomers, a dynamic resolution of (4) can be carried out by crystallisation with entrainment by crystals of the desired isomer; see Barton and Kirby, J. Chem. Soc. (C) (1962) p.806. However, in respect of a total synthesis, this route suffers the disadvantage that racemic narwedine itself is not readily available.
Barton described the use of lithium aluminium hydride to effect the above reduction, however significant amounts of epigalanthamine were also produced, which is undesirable. Reduction of narwedine using the Meerwein-Ponndorf-Verley conditions gave exclusively epigalanthamine. Subsequently it was disclosed in U.S. Pat. No. 5,428,159 that the requisite transformation could readily be achieved using L-Selectride; see Brown and Krishnamurthy, JACS (1972) p.7159. However, this reagent is expensive and only available in pilot plant quantities, and is therefore unsuitable for large scale production; see Rittmeyer, Chimica Oggi (1995) p.51. Alternative reagents disclosed in the prior art are either as esoteric as L-Selectride, or do not afford sufficiently high levels of diastereoselection.